The present invention relates generally to a communications system and in particular, to a method and apparatus for multiple cable access units accessing a cable telephony communications system.
In a cable telephony communications system, frequency division multiplexing (FDM) is employed. FDM allows two or more simultaneous continuous channels to be derived from a transmission medium by assigning separate portions of the available frequency spectrum, separated by some minimal channel spacing within a block of spectrum, to each of the individual channels. FDM provides for a fixed number of physical channels (i.e. separate frequencies) in a cable telephony communications system. To provide more channels to increase the number of users that may operate on a cable telephony communications system, time division multiplexing (TDM) is used. TDM provides time division multiple access (TDMA) in which users share a carrier frequency in the communications system by being assigned and using one at a time, for a limited amount of time, time division multiplex channels (time slots). In effect, each user gets assigned a different time slot on the same frequency. In this way many users can share the same frequency. Data transmissions are sent in time slots in which a communications unit may be assigned a particular time slot only for a limited amount of time.
One of the challenges of a TDMA system is providing to the users access on demand of the TDMA channels. A related challenge is resolving collisions when two or more users try to access or use the same TDMA channel. Consequently, it is desirable to have a system that elegantly provides access on demand of the TDMA channels, and elegantly resolves collisions when multiple users attempt to use the same channel.